1. Field of the Invention
The present invention relates to an X-ray imaging apparatus and a measurement method and, more particularly, to a technique for measuring delays associated with X-ray output operation.
2. Description of the Related Art
An X-ray imaging apparatus is known, which uses a flat panel detector including a two-dimensional array of detecting elements and TFTs formed from an amorphous silicon and polysilicon, as materials that are deposited and formed on a glass substrate. Such apparatuses vary in type, but are generally configured such that when X-rays strike the flat panel detector, the phosphor wavelength-converts X-rays into visible light. The detecting elements convert the converted light into charges and store the charges. When the TFTs are turned on for each row, the charges stored in the flat panel detector are sequentially read and converted into pixel values. Using such a flat panel detector, an X-ray imaging apparatus generates an image according to the intensity distribution of X-rays, on the flat panel detector, which are transmitted through an object placed between an X-ray generating apparatus (X-ray source) which emits X-rays and the flag panel detector.
Recently, flat panel detectors capable of capturing moving images as well as still images have been developed.
The following can influence the quality of moving images when capturing a moving image performed by repeatedly capturing still images at a high speed:                the difference between the timing at which the X-ray generating apparatus receives a signal indicating X-ray exposure and the timing at which the X-ray generating apparatus starts X-ray exposure in accordance with the signal (X-ray exposure start delay time), and        the difference between the timing at which the X-ray generating apparatus receives a signal indicating when X-ray exposure has stopped and the timing at which the X-ray generating apparatus stops X-ray exposure in accordance with the signal (X-ray exposure stop delay time).This point will be described with reference to FIGS. 7 to 9. FIG. 7 is a timing chart showing the relationship between an X-ray exposure signal, X-ray intensity, and read in the moving image capturing mode.        
The flat panel detector alternately repeats storing and reading charges originating from X-ray exposure. Referring to FIG. 7, “read” indicates periods during which charges are read from the flat panel detector, “Hi” indicates a period during which charges are read, and “Lo” indicates a period during which no charge is read (charges are stored in the case shown in FIG. 7).
When capturing an image by reading charges, offset correction is generally performed. A technique is known where offset correction is performed for a moving image capturing apparatus including a flat panel detector and driving the apparatus (Japanese Patent Laid-Open No. 2002-301053). FIG. 7 shows a timing chart when an X-ray image is generated by reading charges twice per X-ray exposure, and subtracting an image read without X-ray exposure from an image read after X-ray exposure.
Referring to FIG. 7, “X-ray exposure signal” indicates the transition of an X-ray exposure signal supplied to the X-ray generating apparatus, with a Hi period of the X-ray exposure signal indicating X-ray exposure, and a Lo period of the signal indicating the stop of X-ray exposure. The X-ray generating apparatus (not shown) starts X-ray exposure when the X-ray exposure signal goes Hi, and stops X-ray exposure when the X-ray exposure signal goes Lo. It takes a certain time from the instant the X-ray exposure signal goes Hi to the instant X-ray exposure actually starts (X-ray exposure start delay time). It also takes a certain time from the instant the X-ray exposure signal goes Lo to the instant X-ray exposure actually stops (X-ray exposure stop delay time). Referring to FIG. 7, “X-ray intensity” indicates the transition of the intensity of X-rays actually output from the X-ray generating apparatus. Referring to FIG. 7, reference symbol Ta denotes an X-ray exposure start delay time; and Tb, an X-ray exposure stop delay time. In addition, X-ray exposure is performed during storage of charges.
If the frame rate in moving image capturing is high, the magnitudes of the X-ray exposure start delay time and X-ray exposure stop delay time become large relative to the frame interval. FIG. 8 is a timing chart showing an X-ray exposure signal, X-ray intensity, and read when the frame rate in the moving image capturing mode is high.
As the frame rate in moving image capturing increases, since the charge read time is constant, the charge storage period decreases. Actual X-ray exposure may overlap reading of charges because the X-ray exposure is accompanied by an X-ray exposure start delay time and an X-ray exposure stop delay time. Referring to FIG. 8, the hatched portion indicates a portion where actual X-ray exposure overlaps a charge read. Reference symbol Tc denotes an overlap time.
It is generally necessary to emit a predetermined dose of X-rays to capture an X-ray image. If, therefore, X-ray exposure overlaps a charge read, since the intensity of emitted X-rays is not reflected in the stored charges, the quality of an X-ray image deteriorates.
To emit the predetermined dose of X-rays, it is conceivable to secure a predetermined period of time during which an X-ray exposure signal is set Hi or shorten a “Hi” period of the X-ray exposure signal and increase an X-ray tube current or X-ray tube voltage for the X-ray generating apparatus. As in the latter case, an increase in X-ray tube current in the X-ray generating apparatus will lead to an increase in cost, whereas an excessive increase in X-ray tube voltage will lead to a decrease in the contrast of a captured X-ray image. For this reason, to emit the predetermined dose of X-rays, the technique of securing a predetermined period of time during which the X-ray exposure signal is set Hi is generally used. In this case, in order to increase the frame rate in moving image capturing, it is necessary to minimize the time interval from the end of charge reading to the start of X-ray exposure and the time interval from the end of X-ray exposure and the start of charge reading.
FIG. 9 is a timing chart showing the relationship between an X-ray exposure signal, X-ray intensity, and read when X-ray exposure starts concurrently with the end of charge reading, and charge reading starts concurrently with the end of X-ray exposure.
Referring to FIG. 9, the X-ray exposure start delay time Ta and an X-ray exposure stop delay time Tb are measured in advance. The X-ray exposure signal is set Hi the measured time Ta earlier than the timing of the end of charge reading. In addition, the X-ray exposure signal is set Lo the measured time Tb earlier than the timing of the start of charge reading. Performing this control can maximize the frame rate without making an actual X-ray exposure period overlap a charge read period.
In general, a flat panel detector can be connected to various types of X-ray generators depending on the region to be imaged or the imaging purpose. However, different X-ray generators differ in X-ray exposure start delay time and X-ray exposure stop delay time. For this reason, it is necessary to measure an X-ray exposure start delay time and an X-ray exposure stop delay time in advance for each apparatus to be used.
Japanese Patent Laid-Open No. 62-276798 discloses an arrangement for measuring an X-ray exposure start delay time and an X-ray exposure stop delay time, and correcting the X-ray exposure start timing and the X-ray exposure stop timing by correcting the measured delay times. In addition, Japanese Patent Laid-Open No. 2004-166728 discloses an arrangement in which an X-ray detector different from a flat panel detector is provided outside the flat panel detector to measure a read timing.
With regard to the arrangement disclosed in Japanese Patent Laid-Open No. 62-276798, there is no description about a specific technique of measuring the X-ray exposure start delay time Ta and the X-ray exposure stop delay time Tb. It is generally necessary to additionally prepare an X-ray detector for measuring an actual X-ray intensity and to measure an output from the X-ray detector and an X-ray exposure signal and a read signal with an oscilloscope or the like. This measurement requires much time and labor.
In addition, the arrangement disclosed in Japanese Patent Laid-Open No. 2004-166728 requires an additional X-ray detector outside the flat panel detector to measure the X-ray exposure start delay time Ta and the X-ray exposure stop delay time Tb, and hence has the problem of high cost.